Mercedes, therefore, wants to take it step by step, which is where this hybrid S comes in. Or the S300 Blue Tec Hybrid L, to give it its full name.

It features the same drivetrain as the E300 Hybrid we tested a couple of years ago. Underneath there's a 2.1-litre four-cylinder diesel engine, with a small electric motor sandwiched between engine and seven-speed gearbox, driving the rear wheels.

Before we get to the thorny issue of a four-pot diesel in the car upon which Mercedes-Benz's reputation is built, perhaps some numbers would be more beneficial.

The diesel engine produces 201bhp and 367lb ft of torque, while the little electric motor adds another 27bhp and 192lb ft of torque. And in a car weighing 2035kg, it's nothing short of a miracle that the S300 can accelerate from 0-62mph in just 7.6 seconds and cruise to an electronically limited top speed of 155mph.

I'm frothing with blind rage at the very thought of this car. Calm me down with some economy and efficiency numbers.

Here's the witchcraft: this big, luxurious Mercedes barge will emit just 120g/km of CO2 and a claimed 61.4mpg, making it the most frugal and environmentally friendly S-Class Merc currently builds.

Impressive.

It is. For such a large car to cost only £30 a year in road tax, and offer a claimed range of 944 miles is, frankly, incredible by any measure. The reality is, of course, a bit less fantastic: during our test run - taking in a healthy mix of motorway and town routes - we only managed 44.1mpg. Which is still hardly shabby.

That's superb. But do S-Class buyers really care about fuel economy and emissions?

Perhaps not, but focusing for the moment just on the mechanics of the thing, it's mightily impressive, if not quite as unstressed as we'd hoped. That four-cylinder diesel engine is the weak point here; it's too gruff and loud for something that cossets and cocoons you as an S-Class.

That's not to say it's ear-splittingly intrusive. It's just that it should have been quieter, because you're in an S-Class. If the battery has charge, the S300 will glide off on electric power alone up to around 10mph before the engine kicks in, at which point you'll hear it. There's good isolation from the vibration of the four-pot, but you can still hear it. And under hard acceleration, the engine makes its presence felt just a little too much for a car that prides itself on huge comfort and magnificent refinement.

The S300 will seamlessly transition between engine and electric motor neatly, though - much quicker than the earlier E300 Hybrid we tested - and automatically turn off the engine when you're coasting, running on battery juice alone. It's a neat trick.

Pace is never an issue - you're never going to be drag racing an S-Class anyway - and it's got good shove up and down the range. It feels punchy rather than thunderous, but in day-to-day situations you're never left wanting. The usual S-Class handling nuances - light steering, benchmark ride comfort - are all present and correct too. It's a fabulous thing to waft around in.

So what should I do? The figures are impressive, but the world will know I've bought Merc's biggest car with one of its smallest, noisiest engines.

That's the key. There's also the price of the thing. This car weighs in at £72k (our test car tipped the scales at over £100k with options!), while the S350 V6 diesel - faster, more powerful, more torque, around 50mpg if you're careful - starts at £62k, or £66k for the long wheelbase version (which is the only wheelbase available for the S300). There's nothing hugely wrong with this S300, and it's a fine effort. We just wish the engine was more refined. And a V8.